Thank you silver911, that's precisely what I had almost settled on. It's a little unsightly in comparison to the neat Rennline item above though! Great to know its handled a larger phone and without problems. For anyone reading, note there is the Magic Mount range, and the Magic Mount PRO range, and it's the latter that are the newer/stronger ones.
There are two from Socshe's PRO range that appeal:
Though like yourself it is the latter I was thinking of going for, for maximum stability.
Not sure what car you have as you refer to the "top of dashing front of the speaker grille"

but in my 993 I was thinking of mounting it on the
underside of the ashtray, using this approach:
So that it's accessible like this:
Based on your experience, do you think it's strong enough to hold 'upside down'?